Hydrostatic Test Requirements

At first, you have to define the code used in design and construction of your piping system, I mean, did you used ASME Process Piping Code, B31.3 or Gas Transmission and Distribution Piping System, B31.8.

I think that the 3rd party inspector is afraid of doing the hydrotest at ambient temperature which may cause the pipe skin temperature to be lower than the MDMT of the whole piping system. If that is the problem, I like to declare that no way to allow for your piping system to operate or tested at temperatures lower than its MDMT.

So, the 3rd party inspector have the right to verify if the system subjected to that case or not. And the Form of Hydrostatic Test Procedure must contain a record for these temperatures.

Notes:

1. If there there is no statement in the contract of the project to proceed a hydrostatic test (by water or another liquid), the ASME codes states that gas pipelines can be pneumatically tested (by air or gas) in lieu of hydrostatic test by the conditions and details specified by the code.

2. The relation between design pressure P_d and hydrostatic test pressure P_h for a piping system designed and constructed as per ASME B31.3 is: P_h = 1.5 P_d (S_{at ambient temp.} / S_{at design temp.}). But if your piping system is designed and constructed in accordance with ASME B31.8, the relation between test pressure and design pressure is as indicated in Table 841.322(f).

3. For more understanding to MDMT, see the following CR4 Threads: Minimum Design Metal Temperature (MDMT) & Impact Test.

4. Relation between Stresses and holding time: The following article is extracted from Pressure Piping code B31.8, Gas Transmission and Distribution Piping System, Para. 841.32 (Edition 1995):

Quoted

Para. 841.321 All pipelines and mains to be operated at a hoop stress of 30% or more of the SMYS of the pipe shall be given a test for at least 2 hours to prove strength after construction and before being placed in operation.

Para. 841.322

(a) Pipelines located in Location Class 1 Division 1 shall be tested hydrostatically to 1.25 times design pressure if the max. operating pressure is greater than 72% SMYS.

(b) Pipelines located in Location Class 1 Division 2 shall be tested either with air or gas to 1.1 times the max. operating pressure or hydrostatically to at least 1.1 times the max. operating pressure if the max. operating pressure is 72% SMYS or less.

(c) Pipelines and mains in Location Class 2 shall be tested either with air to 1.25 times the max. operating pressure or hydrostatically to at least 1.25 times the max. operating pressure.

(d) Pipelines and mains in Location Class 3 and 4 shall be tested hydrostatically to a pressure not less than 1.4 times the max. operating pressure. This requirement doesn't apply if, at the time the pipeline or main is first ready for test, one or both of the following conditions exist

(1) the ground temp. at pipe depth is 32 ^oF or less, or might fall to that temp. before the hydrostatic test could be completed, or

(2) water of satisfactory quality is not available in sufficient quantity.

In such cases an air test to 1.1 times the max. operating pressure shall be made, and the limitations on operating pressure imposed by Table 841.322(f) do not apply.

(e) The test requirements given in Para. 841.322(a), (b), (c), and (d) are summarized in Table 841.322(f).

(f) In selecting the test level, the designer or operating company should be aware of the provisions of Para. 854 and the relationship between test pressure and operating pressure when the pipeline experiences a future increase in the number of dwellings intended for human occupancy.

Table 841.322(f)-Edition 1999

Test Requirements for Pipelines and Mains to be Operated at Hoop Stresses of 30% or more of the SMYS of the pipe

* Note 1. For exceptions, see Para. 841.322(d).

m.o.p. = max. operating pressure (not necessarily the max. allowable operating pressure)

d.p. = design pressure

t.p. = test pressure

Unquoted